Maduro started his career as a bus driver in Caracas, rose through the ranks as a union leader and eventually became part of Chavez's inner circle. He has been the interim leader since Chavez's death, and served as vice president and foreign minister in his administration.

Capriles is the former governor of Miranda state. He lost to Chavez in October's presidential vote, coming within 10 percentage points of the longtime leader. It was a significant gap, but the closest any opposition candidate ever came to defeating Chavez during his rule.

Photos: Political career of Hugo Chavez 18 photos

Photos: Political career of Hugo Chavez18 photos

Political career of Hugo Chavez – Army Lt. Col. Hugo Chavez, who led a 1992 attempted coup, speaks to reporters on March 26, 1994, after he was freed from jail. Chavez was freed after charges were dropped against him for leading the first of two attempted coups against the government of former President Carlos Andres Perez, who was later removed from office.

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Photos: Political career of Hugo Chavez18 photos

Political career of Hugo Chavez – Venezuelan president-elect Chavez visits Bogota, Colombia, on December 18, 1998. On December 6, Chavez had been elected the youngest president in Venezuela history.

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Photos: Political career of Hugo Chavez18 photos

Political career of Hugo Chavez – President Chavez greets supporters with his then-wife, Marisabel Rodriguez de Chavez, beside him as he arrives to preside over a parade in his honor on February 4, 1999, in Caracas. Chavez was sworn in as president on February 2.

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Photos: Political career of Hugo Chavez18 photos

Political career of Hugo Chavez – Chavez inspects military maneuvers of the national Air Force on March 17, 2001, in Catilletes near the border with Colombia. In June 2000, Chavez was re-elected to the presidency for a six-year term, under the new constitution created by his government in 1999.

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Photos: Political career of Hugo Chavez18 photos

Political career of Hugo Chavez – People try to take shelter from gunshots fired near Altamira Square in Caracas on August 16, 2004. At least three people were wounded by gunshots after Chavez supporters fired on opposition demonstrators, police said. A vote to recall Chavez as president failed on August 15.

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Photos: Political career of Hugo Chavez18 photos

Political career of Hugo Chavez – Chavez, left, stands in front of supporters with Fidel Castro of Cuba, center, and Evo Morales of Bolivia, right, during a rally at the Plaza de la Revolucion in Havana, Cuba, on April 29, 2006.

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Photos: Political career of Hugo Chavez18 photos

Political career of Hugo Chavez – Chavez meets with Pope Benedict XVI at his private library on May 11, 2006, in Vatican City.

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Photos: Political career of Hugo Chavez18 photos

Political career of Hugo Chavez – Chavez embraces Iranian President Mahmoud Ahmadinejad, right, in Tehran, Iran, on July 1, 2007. The two presidents have enjoyed a close relationship and Chavez has referred to Ahmadinejad as his "ideological brother."

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Photos: Political career of Hugo Chavez18 photos

Political career of Hugo Chavez – Chavez speaks during a rally in Caracas on November 18, 2008. Chavez pushed to change term limits in Venezuela through a referendum that passed on February 15, 2009, clearing the way for him to run for a third six-year term.

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Photos: Political career of Hugo Chavez18 photos

Political career of Hugo Chavez – Chavez, right, gives a copy of the book, "The Open Veins of Latin America" by Eduardo Galeano to President Barack Obama during a multilateral meeting at the Summit of the Americas in Port of Spain, Trinidad, on April 18, 2009.

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Political career of Hugo Chavez – Chavez, right, greets Russian Prime Minister Vladimir Putin during his visit to the presidential palace in Caracas on April 2, 2010.

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Photos: Political career of Hugo Chavez18 photos

Political career of Hugo Chavez – Chavez salutes to the audience after passing a law in Caracas on November 12, 2011. Chavez has undergone several rounds of cancer treatment in Cuba, beginning in 2011.

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Political career of Hugo Chavez – Chavez participates in a ceremony at the Miraflores presidential palace in Caracas on January 27, 2012.

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Photos: Political career of Hugo Chavez18 photos

Political career of Hugo Chavez – Chavez, left, jokes with American actor Sean Penn, right, during his visit to Miraflores presidential palace in Caracas on February 16, 2012. Penn thanked Chavez for the support given by the Venezuelan government to his nongovernmental organization, which benefits victims of the 2010 Haiti earthquake.

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Political career of Hugo Chavez – Chavez acknowledges supporters on the streets of Caracas while on his way to the airport to travel to Cuba for ongoing cancer treatment on February 24, 2012.

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Photos: Political career of Hugo Chavez18 photos

Political career of Hugo Chavez – Chavez gestures to the crowd during his closing campaign rally in Caracas on October 4, 2012. The leftist leader won a fourth term on October 7, extending his presidency to 2019.

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Photos: Political career of Hugo Chavez18 photos

Political career of Hugo Chavez – A handout picture released by the Venezuelan presidential press office on Friday, February 15, 2013, shows Chavez surrounded by his daughters and holding the February 14 edition of the official Cuban newspaper Granma at a hospital in Havana, Cuba.

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Political career of Hugo Chavez – Chavez supporters gather in Caracas' Bolivar Square to mourn Chavez's death on March 5, 2013.

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Adiós a Hugo Chávez

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Hugo Chavez's legacy

Maduro has pledged to continue Chavez's plan to build "21st century socialism." Capriles says he pushing a more moderate approach, promising to continue social programs and improve the country's economy.

Crime, inflation and food shortages are key problems on the minds of many in the nation. But heated rhetoric on the campaign trail has focused more on personal attacks between the candidates and sparring over the legacy of Chavez.

No matter who wins, the question of what will happen in Venezuela without Chavez at the helm looms large over the elections.

How many people are expected to cast ballots?

More than 18.9 million Venezuelans are registered to vote in the presidential election.

Venezuela's National Electoral Council estimates that more than 100,000 Venezuelans will vote abroad at diplomatic offices in 88 countries. There are more than 37,681 Venezuelans registered to vote in the United States, according to government figures.

Who will be watching?

In addition to representatives from national organizations serving as observers, Venezuela's National Electoral Council says 170 foreigners have been invited to witness the elections. The international group includes delegations from the Union of South American Nations and the Atlanta-based Carter Center.